A contribution to the study of kotoite: data on three Romanian occurrences
published: Jun 8, 2004
Samples of kotoite from three boron-bearing magnesian skarns in Romania (Ba/iso-ents/isodia/breve.gif___brevei¸ta Bihor, Pietroasa and Cacova Ierii) were investigated in order to add new data on this rare mineral species and to assess the influence of the chemical variation on the main physical and crystallographic constants of the mineral. In all three occurrences, the mineral is present in the outer skarn zones developed at the contact of igneous bodies of Upper CretaceousPaleocene age with dolomite sequences. The grains of kotoite are typically rounded and coated by thin seams of brucite. The compositional limits are given by 0.006 to 0.018apfu Fe and 0.001 to 0.018apfu Mn at Ba/iso-ents/isodia/breve.gif___brevei¸ta Bihor, 0.016 to 0.017apfu Fe and 0.003 to 0.004 apfu Mn at Pietroasa and 0.046 to 0.063apfu Fe and 0.004 to 0.007apfu Mn at Cacova Ierii. All the analyzed samples are Ca-poor. The increasing (Fe,Mn)-for-Mg substitution in kotoite causes an increase of the density, mean index of refraction, a cell parameter and cell volume; on the contrary, the c cell parameter decreases with increasing weighed ionic radius of the six-fold coordinated cations, then decreases with advancing (Fe,Mn)for-Mg substitution. It is reasonable to assume that in all three occurrences kotoite crystallized in the temperature range of 400575 C, at pressures of up to 3 kbar. Subsequent alteration in szaibelyite+brucite or, more frequently, in brucite+sassolite, was influenced by the pH of the late hydrothermal solutions.